The present project will examine how effector proteins of bacterial pathogens attack plants and cause pathogenic outcomes, and how the plant counters these attacks. My recent work on Arabidopsis defense responses, findings from labs cataloguing Pseudomonas syringae effector proteins, and work on bacterial pathogens of mammals all point to the relevance of ADP-ribosylation in plant-bacteria interactions. This is a new, largely unstudied area of plant pathogenesis. Three Specific Aims are proposed: 1) The contributions of recently implicated plant ADP-ribose pyrophosphatase and plant poly(ADP-ribose) glycohydrolase to plant disease resistance will be examined. 2) Bacterial ADP-ribosylation of plant host targets will be studied. The ADP-ribosylation activity of three P. syringae ADP-ribosyltransferase effector proteins will be determined, the respective contributions of these effectors to virulence will be examined, and their plant protein targets will be identified. 3) The role of endogenous poly(ADP-ribosyl)ation in plant defense and HR cell death will be investigated. Plant diseases threaten human sustenance and serve as an informative model for host-pathogen interactions research. The proposed work opens new areas highly suitable for future independent research. [unreadable] [unreadable] [unreadable]